The reduction of oxygen on PEDOT-based electrodes
thesisposted on 23.02.2017, 01:51 by Kerr, Robert John Pascoe
Poly(3,4-ethylenedioxythiophene) (PEDOT) is a conducting polymer that has recently been found to exhibit electrocatalytic activity towards the oxygen reduction reaction (ORR) at rates comparable to platinum. The work presented herein aims to further investigate the electrocatalytic behaviour of PEDOT towards the ORR and determine its potential as an electrode material for energy-related applications such as fuel cells and metal-air batteries. The initial focus of this research was to characterise the electrocatalytic behaviour of PEDOT towards the ORR. A rotating ring-disc electrode study on PEDOT electrodes in aqueous 0.1 M KOH solution showed that the ORR on a PEDOT electrode prepared using a chemical vapour phase polymerisation technique proceeds by a 2-electron pathway at low overpotentials and a series 4-electron pathway at higher overpotentials. In contrast, the ORR on a PEDOT electrode prepared using electrochemical polymerisation was found to only proceed by the 2-electron pathway. Subsequent investigations ultimately showed that this discrepancy was the result of residual electrocatalytic iron(II) species that are incorporated into PEDOT during vapour phase polymerisation. In situ Raman and in situ UV-Vis spectroscopy were used in order to characterise the electronic interactions that occur between the iron(II) species and PEDOT. PEDOT-based electrodes were designed for operation in an H2/O2 alkaline membrane fuel cell setup. Structured PEDOT air-electrodes were prepared by depositing various loadings of PEDOT onto a multi-walled carbon nanotube (MWCNT) buckypaper, a microporous layer carbon paper (MPL-CP), and a carbon paper (CP). The electrodes were characterised using SEM imaging and EDS. The PEDOT air-electrodes were evaluated in real operating conditions using an AMFC tester. The small pore size of the MPL-CP and MWCNT substrates led to pore blockage at low PEDOT loadings. A combination of increased hydrophilicity and a more open pore structure led to severe flooding of PEDOT/CP electrodes. A modest power output of 11 mW cm-2 was achieved in a cell that also suffered from non-optimised ionomer loadings and operating conditions. It is apparent from this work that the low electrocatalytic activity of PEDOT towards the ORR may prevent it from finding application in fuel cells. The ORR on PEDOT electrodes was measured in the aprotic ionic liquid [P6,6,6,14][Cl], which has shown promise as an electrolyte for magnesium-air batteries. The electrocatalytic activity of PEDOT was found to be inferior to that of platinum and glassy carbon in terms of the ORR onset and limiting current-density. Upon addition of 2.5 mmol g-1 ethylene glycol to the ionic liquid, however, it was observed that the electrocatalytic activity of PEDOT towards the ORR was similar to that of platinum and glassy carbon. The work presented in this thesis examines various aspects of the ORR on PEDOT electrodes. The discovery of electrocatalytic iron(II) species that are incorporated into PEDOT films during the vapour phase polymerisation process is a finding that will prove significant for future studies into composite electrodes. While the development of structured PEDOT air-electrodes for fuel cell applications is limited by its low electrocatalytic activity towards the ORR, the performance of PEDOT in the ionic liquid [P6,6,6,14][Cl] with added ethylene glycol is encouraging for metal-air battery applications.